Like the tiny screen shot below, the buttons on line format toolbar are just grey.
How can I activate them?
I don't think it work with part model
You have to be in drawing
You can use the Line Format toolbar in a model, but it's uses are limited. As far as I know you can only use it to change the appearance of sketch entities. For example, if you select a line or lines in an active sketch then some of the icons will become active, and allow you to edit the selected lines., but these changes only apply while the sketch is active. When you close the sketch the lines will revert to the color you have selected for inactive sketches at Tools > Options > Colors. However, if you click on an inactive sketch then some of the icons on the Line Format toolbar will again become active so you can change the appearance.
you will get that option only in drawing not in modelling . In modelling you can change the color of sketch only
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